The proposed studies employ the fetal and newborn lamb as experimental models for the study of thyroid and pituitary-thyroid physiology in the fetal and neonatal periods. Included are studies of the maturation of hypothalamic-pituitary control of thyroid function in the fetus; studies of the mechanism(s) of fetal triiodothyronine deficiency; studies of maturation of iodothyronine deiodinating systems in the fetus and newborn; studies of the mechanism(s) of neonatal thyroidal hyperactivity; and studies of the role of thyroid hormones in neonatal thermogenesis. BIBLIOGRAPHIC REFERENCES: Sack, J. and Fisher, D.A.: Tyrosine hydroxylase mediates the T3 and thermogenic responses to parturition. Clin. Res. 23: 161A, 1975 (abstr.) Fisher, D.A., Sack, J., Lam, R.W. and Wang, C.C.: TSH metabolism in the newborn lamb. Clin. Res. 24: 166A, 1976 (abstr.)